Oxygen as a biological molecule Oxygen (O 2 ) is perhaps the single most important molecule for the maintenance of life on Earth.  e geological record indicates that our planet’s atmospheric O 2 concentration has fl uctuated substantially, and this is thought to be involved in the evolution of a broad array of antioxidant defenses.  is important and reactive molecule fi rst appeared in our atmosphere over 2.2 billion years ago, and millions of years ago may have been as high as 35% of the atmospheric composition. Not until atmospheric O 2 levels had stabilized at around 21% more than 500 million years ago and intracellular mechanisms evolved to utilize O 2 effi ciently and to contain its reactivity, however, did complex multicellular organisms began to proliferate. Because O 2 has a high standard oxidation–reduction (redox) potential, it is an ideal electron acceptor – and is therefore a sink for the capture of energy for intracellular use.  e reactivity of O 2 , however, also has a cost; O 2 is a strong oxidizing agent that strips electrons from bio- logical macromolecules and induces intracellular damage. Unless adequate defenses are present to control and repair the damage induced by its reactive intermediates, O 2 toxicity supervenes.  is is particularly well known to the intensive care unit physician, as prolonged exposure of the human lung to more than 60% oxygen at sea level causes diff use acute lung injury [1] .  e toxicity of O 2 is due to its intermediate species, known as reactive oxygen species (ROS), which are nor- mally scavenged by multiple cellular antioxidant systems present in both prokaryotic cells and eukaryotic cells. Although O 2 ’s role as an intracellular electron acceptor in respiration has been understood for more than 100 years and the cell’s main defense mechanisms against O 2 ’s toxic eff ects were discovered more than 50 years ago, we are currently entering a new era of understanding how O 2 and ROS operate as cell signal transduction mechanisms in order to maintain intracellular homeostasis and to adapt to cell stress.  e present review is focused on O 2 ’s capacity, acting through such reactive intermediates, to modulate signal transduction. Oxyg en utilization and metabolism Approximately 90 to 95% of the O 2 consumed by the body is utilized by mitochondria to supply cellular energy through respiration and oxidative phosphorylation [2,3]. Oxidative phosphorylation conserves energy from the breakdown of carbon substrates in the foods we ingest in the form of ATP, which is vital for cell function. To generate ATP by aerobic respiration, O 2 is reduced to water in a four-electron process without the production of ROS. ATP is then hydrolyzed to ADP, providing energy to perform basic cellular functions such as the maintenance of ion gradients and the opening of ion channels for nerve conduction, for muscle contraction, and for cell growth, repair, and proliferation. Energy in the form of ATP is derived from the oxidation of dietary carbohydrates, lipids, and proteins.  e pro- por tion of carbohydrates, lipids, and proteins utilized to produce ATP is cell specifi c and organ specifi c. For example, adult brain cells (in the fed state) and erythro- cytes utilize carbohydrates, whereas the energy for Abstract Molecular oxygen is obviously essential for conserving energy in a form useable for aerobic life; however, its utilization comes at a cost – the production of reactive oxygen species (ROS). ROS can be highly damaging to a range of biological macromolecules, and in the past the overproduction of these short-lived molecules in a variety of disease states was thought to be exclusively toxic to cells and tissues such as the lung. Recent basic research, however, has indicated that ROS production– in particular, the production of hydrogen peroxide– plays an important role in both intracellular and extracellular signal transduction that involves diverse functions from vascular health to host defense. The present review summarizes oxygen’s capacity, acting through its reactive intermediates, to recruit the enzymatic antioxidant defenses, to stimulate cell repair processes, and to mitigate cellular damage. © 2010 BioMed Central Ltd Clinical review: Oxygen as a signaling molecule Raquel R Bartz 1,2 * and Claude A Piantadosi 1,2,3 REVIEW *Correspondence: bartz001@mc.duke.edu 1 Department of Anesthesiology, Duke University School of Medicine, Box 3094, Durham, NC 27710, USA Full list of author information is available at the end of the article Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 © 2010 BioMed Central Ltd cardiac contraction derives primarily from fatty acid oxida tion [4-6]. Although O 2 is necessary for aerobic ATP generation, ROS can be produced as a by-product of the nonspecifi c transfer of electrons to O 2 by either mito- chondrial electron transport proteins or by nonenzymatic extramitochondrial reactions. Moreover, numerous endo ge nous ROS-producing enzymes utilize molecular O 2 for their reactions.  e production of ROS by some normal and most pathological mechanisms increases as a function of the oxygen concentration in the tissue, which can result in both direct molecular damage and inter- ference with essential redox regulatory events as described later. A diagram of molecular O 2 use by these enzyme systems and the downstream consequences – good and bad – is shown in Figure 1. Because O 2 and its intermediates are highly reactive, elegant but complex systems have evolved to allow for the continuous production of ATP while minimizing ROS production by normal metabolism.  e proteins of the respiratory complexes, for instance, only allow about 1 to 2% of the O 2 consumed by the mitochondrial electron transport system to generate ROS.  is sequence of oxidation–reduction reactions generates a fl ow of electrons through Complexes I to IV of the electron transport system, which produces an electromotive force across the inner mitochondrial membrane used by the ATPase, also known as Complex V, to synthesize ATP. In the process, small quantities of singlet oxygen and superoxide anion ( . O 2 – ) are produced primarily at Complex I and Complex III in proportion to the local O 2 concentration and the reduction state of the carrier. Although such ROS can clearly damage mitochondria and adjacent organelles by oxidizing DNA, proteins, and lipids, or by promoting the formation of adducts with DNA, mitochondria are protected by superoxide dis mu- tase (SOD2) and their own glutathione and peroxidase systems.  e small amount of . O 2 – that mitochondria do produce is quickly converted to hydrogen peroxide (H 2 O 2 ), some of which escapes to the cytoplasm and participates in intracellular signal transduction. In fact the majority of ROS-induced cell signaling research has focused on catalytic changes induced by the oxidation of cell signaling proteins by H 2 O 2 , which is the main focus of the present review. Oxygen toxicity: reactive oxygen species production As already mentioned, O 2 and its intermediate forms are highly reactive and O 2 concentrations >21% have been known for decades to be toxic to plants, animals, and bacteria [7-9].  e major ROS are produced by sequential single electron reductions of molecular O 2 , including . O 2 – , H 2 O 2 and the hydroxyl radical (Figure 2). Small amounts of peroxyl, hydroperoxyl, and alkoxyl radicals are also produced – as is the peroxynitrite anion, primarily from the reaction of . O 2 – with nitric oxide [10].  ese reactive molecules are short-lived oxidants that react with one or more electrons on intracellular proteins, lipids, and DNA; if left unrepaired and unabated, these molecules can lead to cell death via apoptosis and/or necrosis. Moreover, the release of oxidized or cleaved macromolecules into the extracellular space may have specifi c and nonspecifi c proinfl ammatory eff ects.  e range of molecular damage produced by ROS is rather remarkable, and encompasses, for instance, lipid peroxidation and nitration, protein oxidation and protein nitration, protein-thiol depletion, nucleic acid hydroxy- lation and nitration, DNA strand breakage and DNA adduct formation. To prevent and repair such diverse ROS-mediated cellular damage, a range of mechanisms have evolved that are upregulated during periods of excessive ROS generation – commonly known as oxida- tive stress – including antioxidant and repair enzymes, and which, not surprisingly, are under the control of cellular signals generated by ROS themselves. Although mitochondria are highly effi cient at reducing O 2 completely to water, they are still the greatest in vivo source of intracellular ROS production simply because of the amount of O 2 consumed during oxidative phosphory- lation [11,12]. Mitochondrial ROS generation, however, is increased at higher oxygen pressure levels as well as by mitochondrial damage; for instance, by mitochon drial swelling during the mitochondrial permeability transi- tion, which uncouples oxidative phosphorylation and increases ROS production. Uncoupling does not, how ever, always increase ROS production; indeed, the produc tion of ROS may actually decrease via the expression of uncoupling proteins, which may relieve the electron escape to molecular oxygen.  e extent of mitochondrial ROS generation also varies with the type of tissue and the level of damage to the mitochondria. For instance, rat heart mitochondria normally produce more H 2 O 2 than liver mitochondria [13] and mitochondria of septic animals produce more H 2 O 2 than mitochondria of healthy controls [14]. A key point is that the regulation of tissue oxygen pressure is a critical factor for the control of ROS production, and loss of this regulation in diseases such as sepsis increases the amount of oxidative tissue damage. Prevention of oxidative damage: balancing oxygen utilization and the antioxidant defenses  e generation of ROS under homeostatic conditions is balanced by antioxidant defenses within and around cells, which include both enzymatic and nonenzymatic mecha- nisms. Antioxidant enzymes catalytically remove ROS, thereby decreasing ROS reactivity, and protect proteins through the use of protein chaperones, transition Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 2 of 9 metal-containing proteins, and low-molecular-weight compounds that purposely function as oxidizing or reducing agents to maintain intracellular redox stability.  e fi rst-line antioxidant enzymes, the SODs, are a ubiquitous group of enzymes that effi ciently catalyze the dismutation of superoxide anions to H 2 O 2 .  ree unique and highly compartmentalized mammalian superoxide dismutases have been characterized. SOD1, or CuZn- SOD, was the fi rst to be discovered – a homodimer containing copper and zinc found almost exclusively in the cytoplasm. SOD2, or Mn-SOD, is targeted by a peptide leader sequence exclusively to the mitochondrial matrix, where it forms a tetramer [15]. SOD3, or EC- SOD, the most recently characterized SOD, is a synthesized copper and zinc-containing tetramer with a signal peptide that directs it exclusively to the extra- cellular space [16].  e presence of SOD2 helps to limit . O 2 – levels and location; within the mito chondrial matrix, for instance, the enzyme’s activity increases at times of cellular stress [15].  is isoform is required for cellular homeostasis, and SOD2 knockout mice die soon after Figure 1. Molecular oxygen use by enzyme systems leading to reactive oxygen species production and downstream consequences. Oxygen (O 2 ) not only leads to superoxide anion ( . O 2 – ) generation by mitochondria and monooxygenases, but is also required for the enzymatic production of the important signaling molecules nitric oxide (NO) and carbon monoxide (CO). Some oxygen-derived reactive oxygen intermediates such as hydrogen peroxide (H 2 O 2 ) have pluripotent e ects in the cell that are not only detrimental, such as protein and DNA oxidation and lipid peroxidation, but are bene cial and adaptive, for instance by enhancement of the antioxidant defenses. Ask1, apoptosis-signaling kinase 1; Fe, iron; HIF-1, hypoxia inducible factor 1; iNOS/eNOS, inducible nitric oxide synthase/endogenous nitric oxide synthase; ONOO – , peroxynitrite anion; PI3K, phosphoinositide 3-kinase; SOD, superoxide dismutase. ·O2- NADPH Oxidase Xanthine Oxidase iNOS/eNOSMitochondria Mixed Function Oxidases Prostanoid Metabolism O 2 H2O2 NO · · ONOO- O2 Mi d F O NA ti ta no id t id nt hi ne e Pr os t e Prost P t hi ne ne Heme Oxygenase ADPH Xa n ADPH X nth t t h Uncoupling CO Protein nitration Lipid peroxidation DNA oxidation Decreased NO availablity Fe Increased NO availablity Vasodilation Protein S-nitrosylation Mitochondrial biogenesis Extracellular matrix damage Endothelial dysfunction Cellular injury Increased vascular permeability Cell growth, repair, and proliferation Adhesion molecule expression Enhanced anti-oxidant defenses Vascular remodeling Matrix Metalloproteinases Protein/DNA oxidation Lipid peroxidation Kinase activation e.g. Ask1, PI3K, Akt Transcription factor activation e.g. NF-kB, HIF-1, Nrf2 Figure 2. Complete and incomplete reduction of molecular oxygen. The production of speci c reactive oxygen species by single electron additions (e – ). O 2 H 2 O 2 . O 2 - . OH +OH -  2H 2 O  e - e - e - e - 2H + Superoxide anion Hydrogen  peroxide Hydroxyl  radical 2H +       Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 3 of 9 birth and exhibit cardiac abnor malities, hepatic and skeletal muscle fat accumulation, and metabolic acidosis [17].  e product of SOD, H 2 O 2 is usually degraded by peroxidases to prevent subsequent cellular damage; how- ever, H 2 O 2 may also function as a signaling molecule. Although produced in small amounts under homeostatic circumstances, H 2 O 2 production may increase in response to cellular stresses such as infl ammation. For cells to maintain normal H 2 O 2 tone, therefore, other antioxidant defenses have evolved – including two main classes of enzymes. H 2 O 2 is converted to water and O 2 by catalase or to water and an oxidized donor by peroxi- dases, such as the selenium-containing glutathione per- oxi dases. Catalase is sequestered in mammalian cells within the peroxisomes, which can be clustered around the mitochondrial outer membrane [18,19]. Much of the H 2 O 2 produced within mitochondria and diff using past the outer membrane is therefore converted to water and O 2 .  e glutathione peroxidase enzymes couple H 2 O 2 reduction to water with the oxidation of reduced gluta- thione to the glutathione disulphide, which is then reduced back to reduced glutathione primarily by the activity of the pentose phosphate shunt. Glutathione peroxidase isoenzymes are widely distributed in cells and tissues, and are mostly specifi c for reduced glutathione as a hydrogen donor [20]. Mito chondria and certain other organelles also contain other systems to detoxify ROS, including glutaredoxin, thio redoxin, thioredoxin reduc tase, and the peroxiredoxins. Other important enzymes with essential antioxidant and signaling functions are the heme oxygenases (HO-1 and HO-2). HO-1 is the stress-inducible isoform, also called HSP 32, and utilizes molecular O 2 and NADPH to catalyze the breakdown of potentially toxic heme to biliverdin, releasing iron and carbon monoxide. Biliverdin is converted to bilirubin in the cytosol by the enzyme biliverdin reductase. HO-1 is ubiquitous, but levels are especially high in Kupff er cells of the liver, in the lung, and in the spleen. HO-1 knockout mice have anemia and tissue iron accumulation and low plasma bilirubin. HO-1 thus functions to remove a prooxidant (heme) and generate an antioxidant (biliverdin), and the iron and carbon monoxide have important signaling roles, especially during cell stress.  e iron is initially a prooxidant mainly because ferrous iron can donate an electron to acceptor molecules – if this is H 2 O 2 , the hydroxyl radical is generated and causes oxidative stress. If ferric iron can be reduced, the cycle continues (for example, a superoxide-driven Fenton reaction). Ferric iron is not highly reactive, however, and many iron- containing enzymes are inactive in the ferric state. HO-1 knockout mice are therefore susceptible to infl ammation and hypoxia but may actually suff er less lung damage when exposed to 100% O 2 [21], perhaps in part due to the recruitment of iron defenses such as ferritin. HO-1 induc tion, however, provides protection against ischemia– reperfusion injury of the heart and brain, provides protection in severe sepsis, and plays a role in tissue repair and in mitochondrial biogenesis [22-24]. Approaches to capitalize on the benefi cial eff ects of HO-1 induction during periods of oxidative stress in critical illness is an area of active investigation. Nonenzymatic antioxidants such as reduced gluta- thione, vitamin C, vitamin E, and β-carotene also func- tion to protect cells from the damaging eff ects of ROS. Despite a wide range of mechanisms to limit . O 2 – production, over long periods of time ambient O 2 levels of 21% still damage DNA, protein, and lipids. To deal with this molecular damage, inducible repair mechanisms protect the cell from increased ROS production. As noted earlier, however, in many instances the induction of these defenses actually requires oxidative modifi cation of specifi c cell signaling proteins in order to initiate the protective response. In short, the mechanisms that limit the amount of H 2 O 2 and other ROS within the cell must work in a coordinated manner with redox-regulated signaling systems. Peroxi- redoxins, catalase, and glutathione peroxidase are all capable of eliminating H 2 O 2 effi ciently [25,26], but exactly how these many mechanisms are coordinated is not fully understood – although a deeper understanding of the functions of specifi c ROS detoxifi cation enzymes and their interactions with classical phosphorylation-based signal transduction systems is slowly emerging. Intracellular signaling mechanisms from oxygen and reactive oxygen species (hydrogen peroxide) Recent work has indicated that H 2 O 2 is important as a signaling molecule, despite the molecule’s short bio- logical half-life, even though many questions remain unanswered about how it functions.  e major un- resolved issues include how H 2 O 2 gradients or channels are formed and main tained in cells and organs in order to regulate protein function. H 2 O 2 is also generated at the plasma membrane– for instance, by the dismutation of super oxide generated by the NADPH oxidases – where it has important roles in cell proliferation and other vital processes. Because H 2 O 2 readily crosses membranes, some investi gators have suggested that erythrocytes, which are rich in catalase, are cell-protective by func- tioning as a sink for extracellular H 2 O 2 [27]. Because ROS-induced intracellular signaling is complex; investigators have used primary and transformed cell lines that can be easily manipulated to investigate H 2 O 2 ’s contribution to specifi c physiological functions.  e amount of H 2 O 2 required to function as a signaling molecule in various cell lines is an area of uncertainty, Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 4 of 9 but it is generally very low. Low levels of H 2 O 2 generated by the activation of many cell surface receptors, including transforming growth factor-1β, TNFα, granulocyte– macrophage colony-stimulating factor, platelet-derived growth factor, and G-protein-coupled receptors, contribute to redox regulation and signal transduction [28-30]. Intracellular H 2 O 2 targets specifi c proteins and changes their activation states. Many proteins that contain a deprotonated cysteine residue may be redox regulated and susceptible to oxidation by H 2 O 2 ; most cysteine residues of many cytosolic proteins, however, are protonated due the low pH in the cytosol and therefore do not react with H 2 O 2 [31,32].  is eff ect may, however confer some specifi city, and some proteins are directly redox regulated, such as ion channels, p53, and aconitase, either by the thiol mechanism or by changes in the oxidation–reduction state of iron or other transition metals [33]. Exposure to ROS leads to reversible oxidation of thiol groups of key cysteine residues in many downstream proteins, including transcriptional regula tors, kinases, Rho and RAS GTPases, phosphatases, structural proteins, metabolic enzymes, and SUMO ligases. Kinases and phosphatases Kinases phosphorylate downstream proteins in active intracellular signal transduction cascades, usually after the stimulation of a receptor. Kinases may be activated or inhibited by phosphorylation, and several are known to be redox regulated, including prosurvival and pro- apoptotic kinases. For instance, H 2 O 2 indirectly activates the prosurvival kinase Akt/PKB [34]. Akt appears to be necessary for host protection against multiorgan dysfunction from sepsis. Another kinase – apoptosis- signaling kinase-1, a member of the mitogen-activated protein kinase kinase kinase family – activates the p38 and the JNK pathways by directly phosphorylating and activating SEK1 (MKK4)/MKK7) and MKK3/MKK6 [35,36]. Apoptosis-signaling kinase-1 is activated in response to cytotoxic stress and under the presence of H 2 O 2 induced by TNFα in HEK293 cells [37,38].  is kinase is also likely to play a role during sepsis, but how H 2 O 2 manages to stimulate one kinase that is prosurvival versus one that results in cell death is an area of active investigation. Although understanding the nature of redox-based control of kinase activity is in its early stages and how these controls are aff ected during times of severe multisystem stress such as sepsis or trauma is just emerging, it is clear that excessive and nonspecifi c production of H 2 O 2 during periods of oxidative stress interferes with specifi city of redox regulation. Not only are some kinases redox regulated, but their dephos- phorylating protein counterparts (phosphatases) may become inactivated in response to increased intracellular H 2 O 2 . Phosphatases often de-activate specifi c phospho proteins that have been acted on by a kinase. For instance, protein tyrosine phosphatase-1B becomes inactivated in A431 human epidermoid carcinoma cells in response to epidermal growth factor-induced H 2 O 2 production [39]. Insulin-induced H 2 O 2 production also inactivates protein tyrosine phosphatase-1B [40]. Platelet- derived growth factor has been shown to induce oxidation from intra cellular H 2 O 2 and to inhibit the SH2 domain- containing protein tyrosine phosphatase SHP-2 in Rat-1 cells [41]. Phosphatase and tensin homolog is also regulated by H 2 O 2 [42,43]. As a general rule, phosphatase inactivation leads to unopposed activity of the reciprocal kinase; for example, phosphoinositide 3-kinase that activates Akt/PKB, a ubiquitous prosurvival kinase.  e functional requirements for these proteins during times of critical illness are an area of active investigation. Transcription factors Not only does H 2 O 2 regulate certain intracellular kinase and phosphatase pathways, it also interacts with specifi c redox-responsive nuclear transcription factors, co- activators, and repressors. Transcription factors typically become activated in response to signaling cascades activated both by membrane-bound receptors and by intracellular mechanisms. Transcriptional activation of a broad range of gene families are involved in cell survival, cell proliferation, antioxidant defense upregulation, DNA repair mechanisms, control of protein synthesis, and regulation of mitochondrial biogenesis. Among the transcription factors known to be activated in a redox- dependent manner are Sp1, the glucocorticoid receptor, Egr1, p53, NF-κB, NF-E2-related factor 2 (Nfe2l2 or Nrf2), hypoxia inducible factor-1α, and nuclear respira- tory factor-1. Hypoxia inducible factor-1α is a redox- sensitive transcription factor that provides an emergency survival response during severe hypoxic and infl am ma- tory states. Several excellent reviews discuss the impor- tance of these transcription factors and their downstream target genes [44,45]. NF-κB activation and Nrf2 (Nfe2l2) activa tion are also of particular importance in diseases that aff ect critically ill patients. NF-κB is bound in the cytoplasm to IκB in its inactive state [46]. Stimuli that activate NF-κB induce the proteo- somal degradation of IκB, allowing NF-κB to translocate to the nucleus and bind to κB motifs in the promoter region of many genes, including TNFα and inducible nitric oxide synthase (NOS2). H 2 O 2 clearly modulates the function of NF-κB; however, whether its eff ects are inhibitory or activating appear to be cell-type specifi c [47]. H 2 O 2 has been reported to increase the nuclear trans location of NF-κB [48,49], but other studies have shown the opposite eff ect [50]. Although NF-κB regula- tion by ROS is of signifi cant importance during infl am- ma tory states, recent work on other redox-regulated Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 5 of 9 transcription factors such as Nrf2 suggests that H 2 O 2 has pluripotent eff ects. Nrf2-dependent genes are critical for the maintenance of cellular redox homeostasis.  is transcription factor is constitutively expressed in the cytoplasm and is regulated by ubiquitinylation under the dynamic control of kelch- like ECH-associating protein-1 [44,51,52]. In response to oxidative or electrophilic stress, kelch-like ECH-associat- ing protein-1 is oxidized by H 2 O 2 .  is event interferes with Nrf2 ubiquitinylation and its disposal by the proteasome, which allows Nrf2 to accumulate in the nucleus. Nuclear Nrf2 binds to the promoters of genes containing the antioxidant response element consensus sequence [53].  ese genes include hepatic drug- metabolizing enzymes (cytochrome P450 isoforms) and many inducible antioxidant enzymes such as glutathione peroxidase, thioredoxin reductase, and peroxyredoxin-1. Nrf2 also induces HO-1, NAD(P)H quinone reductase-1, and γ-glutamyl cysteine ligase, which help regulate the intracellular redox state [54-57]. A simple schematic of Nrf2 response to mitochondrial H 2 O 2 production is provided in Figure 3. Recent work suggests that Nrf2 transcriptional control plays a signifi cant role in diseases associated with infl ammatory stress [58,59]. Oxidative stress and disease In the healthy body, the ROS production and clearance rates are well balanced. Exogenous sources of oxidants and certain disease states can shift this balance by increasing the amount of ROS produced without adequate detoxifi cation. For example, unchecked oxida- tive stress contributes to the pathogenesis of diabetes and its complications [60-62]. Neurodegenerative diseases, cancer, and aging are all associated with increased rates of ROS generation. Diseases in which acute or chronic infl ammation is a signifi cant component lead to excess extracellular ROS production that may tip the oxidant– antioxidant balance towards acute and/or progressive organ damage, and nonspecifi c ROS production inter- feres with the normal signals generated by ROS. On the other hand, exuberant ROS production in phagocytic cells is critical for protection against microorganisms.  e neutrophil kills bacteria through the induction of NADPH oxidase, which produces a burst of superoxide (oxidative burst). Recent work has also suggested that an H 2 O 2 gradient is necessary for adequate wound healing (for example, in zebra fi sh), but the extent to which such gradients are necessary for mammalian wound healing is still being explored [63]. Figure 3. Schematic of Nrf2 response to mitochondrial hydrogen peroxide production. Hydrogen peroxide (H 2 O 2 )-based molecular signal transduction involving the constitutive Nrf2 transcription factor, which is normally targeted for ubiquitination and degradation (step 1). Various oxidative and electrophilic stresses can stabilize Nrf2 by the oxidation of the kelch-like ECH-associating protein-1 (Keap1) adaptor molecule, allowing free Nrf2 to translocate to the nucleus. The diagram indicates the role of oxidative damage and increased mitochondrial H 2 O 2 production (step 2) in the stabilization of Nrf2 (step 3), and activation of genes that contain the antioxidant response element (ARE) consensus sequence – in this case, superoxide dismutase (SOD2) (step 4). Nrf2 Oxidation Electrophiles Stabilization Keap1 Ubiquitination and degradation ARE NucleusCytoplasmMitochondrion 1 3 SOD2 SOD2 4 H 2 O 2 Oxidative Damage 2 Nrf2 Nrf2 Nrf2 Keap1 Oxidation Nrf2 Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 6 of 9 Oxidative repair (cell protection and proliferation): adaptation, conditioning, and hormesis As mentioned earlier, not all oxidative stress is detrimental to cell survival; in fact, optimal health may require a certain amount of oxidative stress.  e best example is arguably exercise, which induces ROS produc- tion followed by the coordinated upregulation of specifi c antioxidant enzymes, such as SOD2. It has been known for years that exercise induces ROS production beyond basal levels, although the exact rates, species, and quantities are unknown. Moreover, skeletal muscle ROS production during exercise aff ects organs other than the muscles, including the liver, by unknown but probably indirect mechanisms [64].  e idea that exposure to a small dose of a dangerous substance can induce a favorable biological response, long known as hormesis, has been applied to the presumed positive eff ects of H 2 O 2 generated by exercise. Increased skeletal muscle contractile activity has been shown to produce superoxide, nitric oxide, hydrogen peroxide, hydroxyl radical, and peroxynitrite [65-69]. It was once believed that skeletal muscle mitochondria were the sole source of intracellular ROS during exercise [70,71]; however, other sources may derive from the sarcoplasmic reticulum, plasma membrane, or transverse tubules [72,73].  e stresses of muscle contraction during exercise that generates ROS are followed by the upregulation of catalase, protective protein thiols and the SODs [74]. H 2 O 2 diff using across membranes may result in protein/lipid oxidation of nearby cells during exercise [75], but the upregulation of the antioxidant enzymes as well as the redox regulation of mitochondrial biogenesis is probably responsible for many of the benefi ts seen with exercise training [76-78]. Indeed, the administration of large doses of low-molecular-weight antioxidants before exercise interferes with mitochondrial biogenesis in human subjects [79].  ese and similar observations in other model systems off er an explanation for why blanket antioxidant supple- mentation is not the therapeutic panacea that was once hoped. A better understanding of how these molecular pathways are regulated will hopefully lead to new targets to induce intracellular protection and repair pathways during relevant critical disease states. Conclusions Oxygen is fundamental to the aerobic processes of eukaryotic life. Oxygen is consumed within the mito- chon dria to produce ATP, which is hydrolyzed to ADP to provide energy for all intracellular homeostatic and work functions. Because of oxygen’s high chemical reactivity, however, advanced life-forms have had to evolve eff ective mechanisms to limit the biologically-damaging eff ects of O 2 as well as the ability to utilize its intermediates to support cell signaling and damage control during health and disease. In particular, H 2 O 2 has emerged as an important signaling molecule involved in the induction of the antioxidant defenses, cell repair mechanisms, and cell proliferation. Understanding how H 2 O 2 and other ROS are produced, contained, and targeted will open up new avenues of understanding and should lead to novel interventional antioxidant strategies for use in health and disease. Abbreviations HO, heme oxygenase; H 2 O 2 , hydrogen peroxide; NF, nuclear factor; O 2 , oxygen; . O 2 – , superoxide anion; redox, oxidation–reduction; ROS, reactive oxygen species; SOD, superoxide dismutase. Competing interests The authors declare that they have no competing interests. Author details 1 Department of Anesthesiology, Duke University School of Medicine, Box 3094, Durham, NC 27710, USA. 2 Durham Veterans A airs Medical Center, Duke University School of Medicine, 508 Fulton Street, Durham, NC27705, USA. 3 Department of Medicine, Duke University School of Medicine, 200 Trent Drive, Durham, NC 27710, USA. Published: 11 October 2010 References 1. Crapo JD: Morphologic changes in pulmonary oxygen toxicity. Annu Rev Physiol 1986, 48:721-731. 2. Rolfe DS, GC Brown: Cellular energy utilization and molecular origin of standard metabolic rate in mammals. Physio Reviews 1997, 77:731-758. 3. Babcock GT: How oxygen is activated and reduced in respiration. Proc Natl Acad Sci U S A 1999, 96:12971-12973. 4. 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Proc Natl Acad Sci U S A 2009, 106:8665-8670. doi:10.1186/cc9185 Cite this article as: Bartz RR, Piantadosi CA: Oxygen as a signaling molecule. Critical Care 2010, 14:234. Bartz and Piantadosi Critical Care 2010, 14:234 http://ccforum.com/content/14/5/234 Page 9 of 9 . Free Radic Biol Med 2004, 36:1208-1213. 53. Itoh K, Chiba T, Takahashi S, Ishii T, Igarashi K, Katoh Y, Oyake T, Hayashi N, Satoh K, Hatayama I, Yamamoto M, Nabeshima Y: An Nrf2/small Maf heterodimer. cells. Am J Physiol 1995, 269(6Pt 1):L829-L836. 50. Hayakawa M, Miyashita H, Sakamoto I, Kitagawa M, Tanaka H, Yasuda H, Karin M, Kikugawa K: Evidence that reactive oxygen species do not mediate. peroxidation DNA oxidation Decreased NO availablity Fe Increased NO availablity Vasodilation Protein S-nitrosylation Mitochondrial biogenesis Extracellular matrix damage Endothelial dysfunction Cellular